a) The characteristic symptoms of cholera are extensive loss of body fluids and Na+ through continuous debilitating diarrhea that can lead to severe dehydration. If untreated, the disease is often fatal. The underlying cause of the symptoms is the ADP-ribosylation of an arg residue on the a subunit of a GS protein by cholera toxin. Based on the above information, propose a normal function for cAMP in intestinal epithelial cells and suggest a possible treatment for cholera.

b) The physiological effects of epinephrine should in principle be mimicked by the addition of cAMP to target cells, however in practice such additions elicit little or no response. In contrast, when the structural analog dibutyryl cAMP is utilized in the same experiments the expected physiological response is readily apparent. (In dibutyryl cAMP, the 6-amino group of the purine ring is in an amide linkage and the 2' hydroxyl group is in an ester linkage with butyrate.) Explain the reason for divergent behavior of these structural analogs.